High-speed digital circuits generate electromagnetic emissions that must be controlled. Such circuits thus are ordinarily contained in electrically conductive enclosures. However, electromagnetic interference ("EMI") emission is often introduced in the external environment by I/O circuit modules, i.e., circuit modules which transmit data to, or receive data from, a unit outside the enclosure. Such emission can interfere with proper operation of other electronic devices, such as computers, radio communications systems, or television receivers. The Federal Communications Commission ("FCC") has adopted regulations which set forth maximum acceptable limits on EMI emission from computer systems.
In addition to satisfying FCC standards for EMI emission, a computer system and its I/0 circuit module must be adequately protected against EMI from other sources, against electrostatic discharges ("ESD"), and against electrical overstress ("EOS"), all of which are or may be commonly present in the operating environment EMI reception can interfere with proper operation of the electronic components of a computer system. ESD and EOS may contain a wide spectrum of frequencies and can damage those electronic components due to excessive voltages, excessive currents, or both. EOS can introduce large voltages or large currents to the circuit module (as, for example, when I/0 connector cable is accidentally exposed to common 120 volt electric power lines).
To protect the environment against EMI emission from I/O circuit modules in a computer system and to protect the computer system against EMI, ESD and EOS from the environment, the circuitry is typically housed in a conductive cabinet. However, the cable that extends through the cabinet to the outside world is itself a source of EMI, and shielding is thus typically provided for the cable itself. The amount of this cabling can be reduced somewhat by filtering the output of the I/O circuit transceiver. But the best filters for this purpose tend to be expensive, while less-expensive filters, although effective for reducing EMI, also introduce into the cabinet interior circulating currents that can adversely affect the operation of the interior circuitry.